Networking Core Protocols

DNS: Remembering Addresses

DNS operates at the Application Layer, i.e., Layer 7 of the ISO OSI model.
DNS traffic uses UDP port 53 by default and TCP port 53 as a default fallback.
Most popular types of DNS records are below
- A record - The A (Address) record maps a hostname to one or more IPv4 addresses.
- AAAA record - The AAAA record is similar to the A Record, but it is for IPv6
- CNAME record - The CNAME (Canonical Name) record maps a domain name to another domain name.
- MX record - The MX (Mail Exchange) record specifies the mail server responsible for handling emails for a domain.
To lookup ip address of a domain name use the below command.
```
nslookup www.example.com
```

The one who registers the domain name has the authority to set the A, AAAA, and MX records, among other DNS records for the domain.
To view the whois records for any domain
```
whois example.com
```

HTTP stands for Hypertext Transfer Protocol;
S in HTTPS stands for Secure.
This protocol relies on TCP.
HTTP and HTTPS commonly use TCP ports 80 and 443, respectively, and less commonly other ports such as 8080 and 8443.

To connect to a server using telnet
```
telnet <IP_ADDRESS> <PORT>
```
To send a get request after connection
```
GET /flag.html HTTP/1.1
HOST:<HOST_NAME>
```
If you don't have a valid host name send a random text to check if that works.

FTP - File transfer protocol
It is efficient than HTTP for transferring files.
By default, FTP server listens on port 21.
Basic commands in FTP are
- USER - To input a username.
- PASS - To input a password
- RETR - To retrieve or download a file.
- STOR - To store or upload a file.
In Terminal, to connect to the FTP server use,
```
ftp <IP_ADDRESS>
```
Enter the username and password.
Switch to ascii mode type ascii for text files.
List all files using ls.
Download a file using get <FILENAME>.

POP3 - Post Office Protocol version 3
It allows the client to communicate with a mail server and retrieve email messages.
By default, POP3 server listens on TCP port 110.
Basic commands in POP3 are
- USER <username> identifies the user.
- PASS <password> provides the user’s password.
- STAT requests the number of messages and total size.
- LIST lists all messages and their sizes.
- RETR <message_number> retrieves the specified message.
- DELE <message_number> marks a message for deletion.
- QUIT ends the POP3 session applying changes, such as deletions.

IMAP - Internet Message Access Protocol.
POP3 is enough when working with a single device.
IMAP allows synchronizing read, moved, and deleted messages when working with multiple devices.
Some basic commands in IMAP are
- LOGIN <username> <password> authenticates the user.
- SELECT <mailbox> selects the mailbox folder to work with.
- FETCH <mail_number> <data_item_name> Example: fetch 3 body[] to fetch message number 3, header and body.
- MOVE <sequence_set> <mailbox> moves the specified messages to another mailbox.
- COPY <sequence_set> <data_item_name> copies the specified messages to another mailbox.
- LOGOUT logs out.